Devices for photon counting have been proposed and demonstrated for use in connection with various applications. For example, lidar systems, in which light is used to determine the range to and/or characteristics of a target require that photons arriving from the target area be detected, time-stamped, and counted. In general, photon counting devices have included amplifier mechanisms, by which the receipt of a single photon or small number of photons results in the production of a relatively large number of electrons, which can then be passed to processing or detector circuitry. Examples of such amplification devices include avalanche photo diodes (APDs), Geiger-mode APDs, dynodes, and micro channel plates (MCPs) with and without photocathodes. The electrons produced as a result of receiving photons are passed to circuitry to characterize the intensity and/or time of arrival of the photons.
Light detection and ranging (LIDAR) systems have been developed that are capable of remotely measuring winds in connection with weather forecasting and climate studies. In general lidar operates by transmitting light from a laser source to a volume of interest and detecting the time of flight for the backscattered light to determine range to the scattering volume. A Doppler wind lidar additionally measures the Doppler shift experienced by photons scattered back to the instrument due to the motion of molecules and aerosols (e.g. particles and droplets) in the scattering volumes. The speed of the wind is determined from the Line of Sight (LOS) speed of the molecules and aerosols relative to the lidar. However, the range of such systems has been limited, because of the small number of photons that are returned to a detector when ranges are large. As a result, lidar systems are often placed in Low Earth Orbit (LEO) to be relatively close to the surface and therefore travel at a significant speed relative to the surface of the Earth, limiting their ability to economically collect data with the spatial and temporal coverage needed for many environmental and defense applications.
An example of a photon counting system that utilizes an APD, and specifically a Geiger-mode APD, is an imaging ladar (laser detection and ranging) system developed by Lincoln Laboratory. In this system, on the order of 107 electron-hole pairs are produced when a photon is detected. Electrons resulting from the detection event are passed to a latch that causes the output of a counter to be frozen upon detection of a photon. The state of the counter after being frozen encodes the number of clock cycles that have elapsed from the start of counting to the time at which the photon was detected. Accordingly, when used in a lidar application, a time of flight (and thus a range) can be determined. However, because the counter is stopped once a photon is detected, later arriving photons are not counted, and therefore the associated lidar is only useful in connection with the sensing of hard targets. In addition, the system does not provide intensity information on a single flash. Intensity information is only obtained from summing over multiple returns, leading to operating inefficiencies. As another limitation, the Geiger-mode APD requires significant time to quench conduction and recharge bias after a triggering event. In addition, such systems can suffer from high dark noise levels.
Other time-resolved light measurement systems that also utilize APDs provide an analog sample and hold circuit that stores about 5-40 samples. Accordingly, the individual samples are collected over some increment in time, which is dependent on the speed at which a commutator used to distribute returned signals to the elements of the sample and hold circuit is run. As a result, an analog waveform of a return signal can be obtained. The number of samples within the waveform is limited to about 40, because of the size of the analog cells compared to the limited pixel area available for unit cell circuitry in imaging arrays, the need for high bandwidth to achieve high temporal resolution, and the power necessarily dissipated by high bandwidth analog circuits. In addition, such systems are useful only for hard targets, because the noise generated in the high bandwidth analog capture and measurement process must be overcome by the higher return signal power generally available from hard target returns.
Photon counting detector arrays have also been proposed for use in medical imaging applications. According to such systems, detected photons are counted over some period of time. However, no record of the time at which individual counts accumulated within that time period occur is maintained. Accordingly, such systems have application in medical or other imaging applications, but are not capable of providing range information.